Google Nexus and the hidden system update
It is possible that a system update for a Google Nexus device can't be found and - according to this - downloaded, even if the update was definitely released.
A reason for this issue might be the "Google Services Framework" - it forces an error while communicating with the Google update servers. A solution might be to stop the process and delete the data of the framework. The next search request shall be acknowledged by a Google update server - if you're lucky, you're able to download the update afterwards.
In my case the trick helped me with downloading the Android 4.2.2 system update for my Nexus 7.